Drilled shafts, or piers, are often used in the deep foundation industry because they provide an economical alternative to other types of deep foundations. Drilled piers are typically formed by excavating a cylindrical borehole in the ground and then placing reinforcing steel and fluid concrete in the borehole. The excavation may be assisted by the use of drilling fluids, casements or the like. When the concrete hardens, a structural pier suitable for load bearing results. These piers may be several feet in diameter and 50 feet or more deep. They are typically designed to support axial and tensile compressive loads.
Alternatively, driven piles may be used as foundation elements. Particularly in soft soils, where shaft excavation may be difficult due to caving of the soil, driving piling has long been a suitable alternative to drilled-shaft piers. Conventionally, a pre-formed or pre-cast element is driven into the soil using either a high-speed vibratory driving tool or large percussive hammers. Typically, driven piles may be solid pre-cast concrete; solid steel beam; or steel pipe piling. A wide variety of materials and shapes for driven piling is known to those skilled in the art, including tapered piles, I-beams, and the like.
A finished structural foundation element such as a pier or pile has an axial load bearing capacity which is conventionally characterized by components of end bearing (qb) and side bearing, which is a function of skin friction (fs). Loads applied at the top end of the element are transmitted to the sidewalls of the element and to the bottom of the element. The end bearing capacity is a measure of the maximum load that can be supported there, and it will depend on numerous factors including the diameter of the element and the composition of the geomaterial (soil, rock, etc.) at the bottom of the shaft. The side bearing capacity is a measure of the amount of load capable of being borne by the skin friction developed between the side of the pier/pile and the geomaterial. It depends on numerous factors, including the composition of the foundation element and the geomaterial forming the side of the element, which may vary with length (depth). The sum of the end bearing and side bearing capacities generally represents the total load that can be supported by the element without sinking or slippage, which could cause destructive movements for a finished building or bridge atop the foundation.
Although it is desirable to know the maximum end bearing and side bearing for a particular pier or driven pile, it is difficult to make such measurements with a high degree of confidence. Foundation engineering principles account for these difficulties by assigning end bearing and load bearing capacities to a foundation element based on its diameter and depth, the geomaterial at the end of the element and along its side, and other factors. A safety factor is then typically applied to the calculated end bearing and side bearing capacities. These safety factors are chosen to account for the large number of unknown factors that may adversely affect side bearing and end bearing, including geomaterial stress states and properties, borehole roughness generated by the drilling process, geomaterial degradation at the borehole-shaft interface during drilling, length of time the borehole remains open prior to the placement of concrete, residual effects of drilling fluids, borehole wall stresses produced by concrete placement, and other construction-related details. For example, it is common to apply a safety factor of 2 to the side bearing so as to reduce by half the amount calculated to be borne by skin friction. Likewise, a safety factor of 3 is often applied to the calculated end bearing capacity, reflecting the foregoing design uncertainties and others.
The use of safety factors, although judiciously accounting for many of the uncertainties in drilled shaft pier construction and driving piling, often results in such foundation elements being assigned safe load capacities that are too conservative. To compensate, builders construct larger, deeper, and/or more elements than are necessary to safely support a structural load, unnecessarily increasing the time, effort and expense of constructing a suitable foundation.
As a partial solution, it has been known to directly measure the end bearing capacity and skin friction of a drilled-shaft pier. Osterberg (U.S. Pat. No. 4,614,110) discloses a parallel-plate bellows placed in the bottom of the shaft before a concrete pier is poured. The bellows are pressured up with fluid communicated through a pipe coaxial with the pier. Skin friction is determined by measuring the vertical displacement of the pier (corresponding to the movement of the upper bellows plate) as a function of pressure in the bellows. Likewise, end bearing is determined by measuring pressure against the downward movement of the lower bellows plate, as indicated by a rod affixed thereto and extending above the surface through the fluid pipe. Upon completion of the load test, the bellows are depressurized. The bellows may then be abandoned or filled with cement grout, and in the latter case becomes in essence an extension of the lower end of the pier.
The method of Osterberg most often serves only the purpose of load testing. In practice, most often a drilled shaft employing the “Osterberg cell” is abandoned after testing in favor of nearby shafts that do not contain a non-functioning testing cell at their base. The method of Osterberg also is limited to use with drilled shaft piers, because with driven piling, there is no open shaft into which the “Osterberg call” may be placed so that it is positioned beneath the foundation element of interest.
Other methods have been developed for enhancing the load bearing capacity of drilled shaft piers by permanently pressuring up the base, but they lack the testing capabilities of the Osterberg cell. For example, it is known to inject pressurized cement grout under the base of concrete piers to enhance load bearing. In post-grouting, the pressurized grout increases end bearing, but neither the resultant increase nor the absolute end bearing capacity can be determined from the pressure or volume of the grout. In some soils, skin friction may also be increased by allowing the pressurized grout to flow up around the sides of the shaft, but this side bearing capacity, too, is not determinable with this technique.